In games that aren't completely about dungeon delving, interior terrains that aren't dungeon walls or caves become soon something that you'd like to have. Above you see a few wall pieces that have been made from balsa wood and various components. I've drawen lines to the balsa wood by pressing hard with ballpoint pen, then painted (first dark, then brown, and last light yellowish gray drybrush). Pay attention to painting process, as the wood may be distorted by the moisture. If you can put it to dry under a weight somewhere where it can still get air, for example between plates with holes, it's easier to keep straight. You can also try to paint a bit by bit and use thicker paint, although thicker paint naturally may lose some details.
How to create a miniature bookshelf
Shelves in the bookshelf and the chest are resin casts - I have several of those both. Bookshelf itself is quite easy to make from pieces of cardboard (You have read the tutorial, right?). Without making a mold and casts, you can make the shelves by cutting another piece of cardboard and glueing it as seen in the picture a bit lower.
After it, you can cut and glue various round pieces (from plastic stripes sold separately for modelling purposes, cocktail sticks etc) to the support piece. Glue the whole thing black and then paint the backs of the books with varying darkish colors (blue, dark red, brown etc). You can also make shelves with skulls, chrystal balls and similar other items if you want.
Other items in the walls
Fireplace has been made from plaster that has been cast to a mold made from silicon rubber. I've used heavy and durable dental plaster that you may get from some dentists who make dental prosthesis. Map is painted/drawn on yellowish -brown paper (you can also paint it), desk, tables and benches are 1mm cardboard and two decorations on wall (portrait and mirror) are from a button store; I've cut the border of the mirror from cereal box -cardboard.
That's quite about it; Not too complicated but takes a little time. However, you'll find making such pieces worth it if you have frequently interior battles in your games.